1. Field of the Invention
The present invention relates to directional couplers and more particularly relates to directional couplers that are preferably used in, for example, wireless communication devices that perform communication using high-frequency signals.
2. Description of the Related Art
The directional coupler described in Japanese Unexamined Patent Application Publication No. 8-237012 is a known example of a conventional directional coupler. This directional coupler is formed by stacking a plurality of dielectric layers, on which coil-shaped conductors and ground conductors have been formed, on top of one another. Two of the coil-shaped conductors are provided. One of the coil-shaped conductors forms a main line and the other coil-shaped conductor forms a sub-line. The main line and the sub-line are electromagnetically coupled with each other. Furthermore, the coil-shaped conductors are interposed between the ground conductors in the direction in which the layers are stacked. A ground potential is applied to the ground conductors. In the above-described directional coupler, when a signal is input to the main line, a signal is output from the sub-line, the signal having a power that is proportional to the power of the input signal.
However, there is a problem with the directional coupler described in Japanese Unexamined Patent Application Publication No. 8-237012, in that the degree of coupling between the main line and the sub-line becomes higher as the frequency of a signal input to the main line increases (that is, the degree of coupling characteristic is not constant). Consequently, even if signals having the same power are input to the main line, if the frequencies of the signals vary, the powers of the signals output from the sub-line will also vary. Therefore, it is necessary that an IC, which is connected to the sub-line, have a function of correcting the power of a signal on the basis of the frequency of the signal.